Toss the parsnips, garlic, and olive oil on a baking sheet and season with salt, to taste. Roast until the edges are golden and caramelized, about 20 to 25 minutes. If a check during roasting reveals the bottom of the parsnips are cooking faster than the top, give them a toss and return the baking sheet to the oven.
When the parsnips and garlic are roasted, put them into a food processor with the butter, 1/4 cup of the heavy cream and a pinch of nutmeg. Blend the mixture until smooth, and if needed add a bit more heavy cream. Season with salt and pepper, to taste. Transfer the puree to a serving bowl and serve.